Depending on the speed of image acquisition, one or both eyes were imaged in the controls. When only one eye was imaged, it was automatically selected for further analyses. However, when images from both eyes were acquired, the images from the left and right eyes were selected in an alternating manner for further analysis. In the patients with glaucoma, only the eye studied by our laboratory in previous studies was imaged and analyzed. In the absence of any comorbid factors, our laboratory selects the eye with the worse glaucomatous defect as the study eye.
Extracting OCT Enface Images
Eight-micron-thick enface images were extracted from the OCT volume scans. The enface images were sampled axially from 14 to 22 μm below the inner limiting membrane. From a preliminary survey, we determined that sampling at this depth yielded enface images that allowed adequate visualization of the nerve fiber bundle reflectance with minimal interruption from putative glial alteration reflections.12 On the enface images from two older controls and two patients with glaucoma, however, there were still appreciable amounts of putative glial alteration reflectances (obscuring the nerve fiber bundle reflectance) even at the 14–22 μm depth at which the enface images were sampled. These four subjects were excluded from the study.
The enface images were processed to compensate for variations in amount of light reaching the retina, such as low-signal B-scans appearing as dark stripes or shadows from floaters, using an attenuation coefficient technique proposed elsewhere.13,14 In brief, the technique assumes that light attenuation is due to light scattering rather than absorption, and that the amount of light reflected back to the detector is proportional to the amount of light scattered. Hence, the attenuation coefficient at a given depth in the retina is assumed to be proportional to the reflectance at that depth.
Quantifying the Retinal Nerve Fiber Bundle Reflectance
A thresholding technique was used to separate the probable nerve fiber bundles from the non-fiber component (Fig. 3). The approach used in this analysis transforms the pixel values to zero in regions of the image where the pixel values are lower than the threshold but maintains pixel values greater than the threshold.
In each image, the value used for the thresholding was computed as the average pixel brightness value below the retinal nerve fiber layer in a zone (250 by 150 pixels) approximating the triangular region2,3 of the raphe, at 36 μm depth from the inner limiting membrane. The size of this region was large enough that the mean pixel brightness computed from the region was not artifactually high due to the contribution of highly reflective blood vessels but still small enough to approximate the triangular region of the raphe.
A grid of 1° by 1° boxes was superimposed on the threshold images. For any given image, the grid began at the foveal reflex (delineated by BSA across all subjects) and the 1° by 1° grid boxes were added vertically and horizontally until the boundary of the image was reached. Approximately 14 by 22 grid boxes covered the retinal region imaged in each subject. The pixel values within each box were averaged and scaled to a fraction index by dividing the average by the highest pixel value possible—255 in our case. The 1° by 1° grid boxes were small enough to detect wedge-shaped reflectance abnormalities and big enough not to be notably impacted by small caliber vasculature.
A lower normative reflectance reference value for each grid box (representing a specific retinal location) was computed at the 5th percentile from the reflectance findings obtained from the older controls. Nerve fiber bundle reflectance abnormalities were identified in the patients with glaucoma when the reflectance at a region on the enface image was below the lower normative reference value.
To assess the performance of the imaging and analysis protocol, we compared the nerve fiber bundle reflectance findings in the patients with glaucoma to perimetric sensitivity findings. Humphrey Matrix (Model 800; Carl Zeiss Meditec, Inc.) perimetric testing was performed in the patients with glaucoma using the 24-2 testing protocol, with a 5° by 5° square sinusoidal grating.
We chose the Matrix stimulus over the Goldmann size III (G-III) stimulus because its size and shape allowed a more compatible comparison with the analysis approach adapted for the reflectance technique. For instance, due to the potential effects of microsaccades and cyclorotation during testing, there is likely a greater spatial correspondence in matching the 25 deg2 area tested by the Matrix stimulus to a 25 deg2 area on the enface image than matching a 0.15 deg2 area tested by the G-III stimulus to 0.15 deg2 area on the enface images. Furthermore, the square-shaped nature of the Matrix stimulus improved our ability to spatially match the reflectance findings with the perimetric findings, given that the sampling of our structural analysis was 1° by 1° square grid boxes.
Visual field testing results were regarded as reliable when false-positive, false-negative, and fixation errors were less than 20%, 16%, and 20%, respectively.15
We initially assessed the impact of aging on the reflectance of the nerve fiber bundles. Because each subject’s enface image was covered by approximately 14 by 22 grid boxes (with each grid box representing the reflectance at a particular retinal location), we summarized the control groups’ data by computing the median reflectance at each grid-box location across the subjects in the control group under consideration. We also computed the variance in similar manner.
We assessed the effect of aging using two analyses. In one analysis, we used a t-statistic to compare the median reflectances in the younger and older controls. In addition, at each grid-box location, we computed the difference between the median reflectance findings for the younger and older controls. In step two, we used a t-statistic to compare the mean of variance findings in the younger controls to the older controls. We hypothesized that if aging substantially affects the nerve fiber bundle reflectance, then the mean of the variances in the younger controls should be smaller than that of the older controls given the more homogenous age range of the younger controls. We also computed the difference (at each grid-box location) between the variance of reflectance findings for the younger and older controls.
We assessed the performance of the reflectance technique by comparing its findings to the findings from perimetric testing in the patients with glaucoma. In a first analysis, we compared the total deviation (TD) at the perimetric testing locations to the fraction of corresponding enface locations showing reflectance abnormality. The retinal region tested by the imaging and analysis protocol approximately corresponded to six perimetric testing locations on the 24-2 perimetric protocol. In a second analysis, we compared the mean deviation (MD) of perimetric testing to the fraction of the entire enface image showing reflectance abnormality (at P < .05). The strengths of the relations were quantified using Spearman’s rho.
The signal strengths of OCT enface images analyzed were similar for all three groups (Table 1). Contour plots for the reflectance findings of the younger and older controls groups were similar (Fig. 4), and a t-statistic comparing the median reflectance findings for the two groups found slightly lower values (Fig. 5A) for the older group t(614) = −1.72, P = 0.04, Cohen’s d = 0.14. The plot in Fig. 5B shows the difference between the variance of the reflectance findings for the younger controls and the older controls. The median of the difference was zero and a t-statistic comparing the variances between the two groups found the variances to be similar—t(614) = 0.30, P = 0.38.
An example of the reflectance findings in a patient with glaucoma that had good agreement with the perimetric finding is shown in Fig. 6. Fig. 7 shows the relation between the perimetric total deviation at the individual perimetric locations and the fraction of the corresponding enface area, which showed reflectance abnormality, for the patients with glaucoma. Spearman’s rho found a strong relation between the two findings—rs(174) = −0.72, P < .001. Fig. 8 shows agreement between the perimetric mean deviation and the fraction of the entire enface area studied showing reflectance abnormality. A Spearman’s rho found a strong relation between the two measures rs(29) = −0.71, P < .001.
In this study, we describe a clinically applicable imaging and analysis protocol for identifying retinal nerve fiber bundle abnormalities in the region of the temporal raphe using the reflectance properties of the nerve fiber bundles. We evaluated the performance of the reflectance technique by comparing the nerve fiber bundle reflectance findings to Matrix perimetric findings and found a generally good agreement. In the patients with glaucoma, in almost all the perimetric locations where perimetric sensitivity was abnormal, the reflectance finding in the region was abnormal. However, it was not uncommon to find locations of substantial reflectance abnormality with mild to no perimetric abnormality.
The nerve fiber bundle reflectance patterns quantified in controls (Fig. 4) appear consistent with the overall patterns of the nerve fiber bundles.2,3,5 At any given point along the raphe line (y = 0), as the vertical distance from the raphe increases, the reflectance index increases, consistent with the adding-on of new fibers to the nerve fiber bundles as the bundles travel toward the disc.2 We concluded that the analysis approach captured the reflectance signature of the nerve fiber bundles and has promise for identifying regions of nerve fiber bundle reflectance abnormality in glaucomatous degeneration.
The reflectance findings in the younger controls were slightly higher than in the older controls (Fig. 5A), but the effect size was small. Consistent with the small aging effect, although the age range of the older controls was more heterogeneous than the age range of the younger controls, the variance of the reflectance findings within the two groups was similar (Fig. 5B). We deduce that in the absence of significant media opacities, age-adjusted models may not be needed in generating a normative database for analyzing nerve fiber layer reflectance abnormalities in OCT enface imaging.
Generally, there was a strong correlation between the fraction of enface image surface area showing reflectance abnormality and the perimetric findings (Fig. 7). This finding, together with the rarity of finding perimetric locations with substantial total deviation without a corresponding reflectance abnormality, provides a basis for the use of enface imaging for targeted perimetry.16 Thus, in the raphe, the chances of finding perimetric abnormality where the reflectance findings are within normative range is likely to be low.
The ability to resolve the individual nerve fiber bundles in the raphe makes it a potential site for evaluating early glaucomatous abnormality. Abnormality in the raphe can be quantified using changes in the overall reflectance of the nerve fibers bundle/layer (such as done in this study), changes in the width of the nerve bundles, or an increase in the potential gap between the superior and inferior raphe fibers reported elsewhere as “raphe gap”.4 These alternative ways of determining abnormality in the raphe make the region a potentially viable location for assessing abnormality when a diffuse nerve fiber bundle abnormality is suspected in a subject who otherwise has no localized wedge or arcuate defects in the peripapillary RNFL enface reflectance profile.17
As was reported in some previous studies,3,5,18,19 we also found a noticeable deviation of the raphe from the horizontal midline (Fig. 2) in some subjects. Clinically, this implies that it is possible for glaucomatous defects such as nasal steps to cross the horizontal midline, as the raphe does not always perfectly align with the horizontal midline.20 For studies investigating the impact of the retinal anatomical features on the trajectories of the nerve fiber bundles, it will be helpful to consider the impact of the raphe angle on the variability of the RNFB trajectories. The effects of cyclo-rotation may have artifactually impacted the angles shown in Fig. 2. Nonetheless, these angles provide a sense of the extent to which the raphe can deviate from the horizontal midline, and the variability that could exist between subjects during routine clinical testing.
In its current form, the nerve fiber bundle reflectance quantifying technique proposed in this study has some limitations. For example, the technique does not include a method for differentiating between non-retinal nerve fiber bundle reflectance sources such as putative glial alteration reflectances and retinal nerve fiber bundle reflectance.21,22 Further study will be needed to develop methods to mitigate the impact of non-bundle reflecting sources before the method can be widely used. Also, some previous studies have demonstrated the directional reflectivity property of the nerve fiber bundles. It will be useful to investigate how the directional reflectivity of the nerve fiber bundles affect OCT enface imaging of the temporal raphe.23
In conclusion, we developed a clinically applicable protocol for quantifying retinal nerve fiber bundle abnormality in the region of the retinal temporal raphe. The ability to quantify structural abnormality in the raphe, where the conformation of the bundles appears suitable for the early detection of glaucoma, opens the possibility of investigating the temporal raphe as a potential site for the early diagnosis of glaucoma.
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© 2018 American Academy of Optometry
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